Class 9 Science Chapter 5: Measurement of Length and Motion – Complete Question Bank with Answers

Chapter 5 questions directly feed into Class 9 periodic and annual exams, and form prerequisites for Class 10 Physics. The CBSE board pattern for this chapter typically allocates 8–10 marks in theory papers, distributed as: 1–2 MCQs (1 mark each), 1–2 short-answer questions (2 marks each), and 1 long-answer question (3–5 marks). Understanding SI unit standardization (metre, kilogram, second) is critical because all subsequent motion calculations depend on consistent unit usage. Board examiners often ask questions that blend concept recall (e.g., 'Define standard unit') with calculation skills (e.g., 'Calculate speed given distance and time'). Additionally, motion classification — distinguishing rectilinear, circular, and periodic motion — appears in both objective and descriptive formats. Questions testing speed-distance-time relationships frequently employ real-world contexts (vehicle motion, sports, astronomy), requiring students to interpret data and apply the formula v = d/t. This question bank mirrors official board templates and sample papers, ensuring you practise exactly what examiners test.

MCQs test quick recall and basic conceptual clarity. Each of the following questions carries 1 mark. **Q1.** Which of the following is a fundamental unit in the SI system? (A) Metre (B) Centimetre (C) Kilometre (D) Millimetre **Answer: (A) Metre** — The SI system defines the metre as the standard unit of length; all other length units (cm, km, mm) are derived or non-SI. **Q2.** The motion of the tip of a second hand on a clock is: (A) Rectilinear (B) Circular (C) Periodic (D) Both B and C **Answer: (D) Both B and C** — The tip traces a circular path (circular motion) and returns to the same position every 60 seconds (periodic motion). **Q3.** If an object travels 100 metres in 20 seconds, its speed is: (A) 2 m/s (B) 5 m/s (C) 10 m/s (D) 20 m/s **Answer: (B) 5 m/s** — Using v = d/t: v = 100/20 = 5 m/s. **Q4.** Which of the following is an example of rectilinear motion? (A) Earth revolving around the Sun (B) A train moving on a straight railway track (C) A child on a swing (D) Rotation of Earth **Answer: (B) A train moving on a straight railway track** — Rectilinear motion occurs along a straight line; orbital and oscillatory motions are non-rectilinear. **Q5.** The SI unit of time is: (A) Hour (B) Minute (C) Second (D) Day **Answer: (C) Second** — The second (s) is the fundamental SI unit of time; hours and minutes are derived units.

Short-answer questions test concept understanding and brief reasoning. Each carries 2 marks. **Q1.** Why is a standard unit necessary in measurement? Give one example. **Model Answer:** A standard unit ensures uniformity and consistency in measurements across regions, institutions, and countries, allowing precise communication of data. Without standard units, different individuals or groups might use non-comparable measurements (e.g., one person measuring in handspans, another in feet), leading to confusion and errors. Example: If all scientists worldwide used different definitions of the metre, chemical reactions or engineering calculations would yield different results, making scientific collaboration impossible. **Q2.** Define rectilinear motion and circular motion with one example each. **Model Answer:** Rectilinear motion is motion along a straight line (e.g., a car driving on a straight highway). Circular motion is motion along a circular path (e.g., the motion of a satellite orbiting Earth or the tip of a clock hand). Both are fundamental classifications used to describe and predict object trajectories. **Q3.** An athlete runs 400 metres in 50 seconds. Calculate her speed in m/s and km/h. **Model Answer:** Speed v = distance / time = 400 / 50 = 8 m/s. To convert to km/h: 8 m/s × (3600/1000) = 8 × 3.6 = 28.8 km/h. [Note: Multiply by 3.6 to convert m/s to km/h.] **Q4.** What is periodic motion? Name one phenomenon from everyday life that exhibits periodic motion. **Model Answer:** Periodic motion is motion that repeats itself at regular time intervals. The time taken for one complete repetition is called the period. Example: A pendulum swinging back and forth, oscillation of a spring, heartbeat of an organism, or seasons of a year. The motion of the Moon around Earth also exhibits periodicity with a 29.5-day cycle (lunar month). **Q5.** State two advantages of using SI units. **Model Answer:** (1) SI units are universally standardized and accepted globally, enabling scientists and engineers worldwide to collaborate and compare results without conversion ambiguities. (2) SI units follow a decimal (base-10) system, making conversions between units straightforward (e.g., 1 km = 1000 m; 1 kg = 1000 g), reducing calculation errors and improving efficiency in problem-solving.

Three-mark questions demand deeper conceptual reasoning, calculation steps, and structured explanations. **Q1.** Explain why the motion of Earth around the Sun is both circular and periodic. How do these two classification schemes differ? **Solution:** Earth's motion around the Sun is circular because it follows a roughly circular orbital path centred on the Sun. It is also periodic because Earth completes one full orbit every 365.25 days and repeats this motion consistently year after year. Circular and periodic motion are different classification schemes: circular motion describes the path or trajectory (circular shape), while periodic motion describes the time-based repetition of motion. An object can exhibit circular motion without being periodic (e.g., a spinning top moving with changing speed). Conversely, periodic motion need not be circular (e.g., a pendulum swinging back and forth in a straight line). Earth's orbital motion satisfies both criteria simultaneously. **Q2.** A car travels the first 100 km in 2 hours and the next 100 km in 1.5 hours. Calculate the average speed for the entire journey. **Solution:** Total distance = 100 + 100 = 200 km Total time = 2 + 1.5 = 3.5 hours Average speed = Total distance / Total time = 200 / 3.5 ≈ 57.14 km/h Alternatively, converting to m/s: 57.14 km/h × (5/18) ≈ 15.87 m/s. [Note: Multiply by 5/18 to convert km/h to m/s.] **Q3.** Define and distinguish between speed and velocity. Why is velocity more informative than speed in physics? **Solution:** Speed is the distance travelled per unit time and is a scalar quantity (magnitude only). Velocity is the displacement per unit time and is a vector quantity (magnitude and direction). For example, if an athlete runs 400 metres on a circular track and returns to the starting point in 50 seconds, the speed is 400/50 = 8 m/s, but the velocity is 0 m/s (because displacement is zero). Velocity is more informative because it accounts for direction and net change in position, allowing physicists to predict future location and understand motion comprehensively. In rectilinear motion, velocity can be positive (forward) or negative (backward), providing directional information that speed alone cannot convey. This distinction is crucial for vector analysis and solving complex motion problems in higher classes. **Q4.** List the SI base units for length, mass, and time. Explain why standardization of these units was necessary historically. **Solution:** - SI base unit for length: Metre (m) - SI base unit for mass: Kilogram (kg) - SI base unit for time: Second (s) Historically, different regions and professions used diverse measurement systems (e.g., inches, feet, yards in English-speaking countries; cubits and spans in ancient Egypt). This fragmentation caused significant problems: (1) Trade and commerce became unreliable because merchants could not compare quantities across borders. (2) Scientific discoveries could not be verified or replicated because measurements were incomparable. (3) Engineering projects failed when different builders used different unit interpretations. The establishment of the SI (Système International d'Unités) in 1960 provided a universal, decimal-based system, enabling global scientific collaboration and standardizing technology worldwide.

Five-mark questions require comprehensive explanations, detailed derivations, and multi-part reasoning. **Q1.** (a) Define the metre as the SI unit of length. (b) Explain how the metre is currently defined and why previous definitions were replaced. (c) Why is the metre a fundamental unit and not derived? (d) Convert 5 km into metres and millimetres. **Full Solution:** (a) The metre (symbol: m) is the SI base unit of length, used to measure distances and linear dimensions. (b) The metre's definition has evolved: (i) Originally (1793), it was defined as 1/10,000,000 of the distance from the North Pole to the Equator along the Paris meridian. (ii) In 1960, it was redefined as a multiple of the wavelength of krypton-86 radiation (299,792,458/1,000,000,000 times the wavelength). (iii) Currently (since 2019), the metre is defined using the speed of light: 1 metre is the distance light travels in vacuum in 1/299,792,458 of a second. Previous definitions required reference objects or highly specific conditions, making them inconvenient and prone to error. Modern definitions use universal constants, ensuring consistency and reproducibility anywhere on Earth or in space. (c) The metre is a fundamental (base) unit because length cannot be expressed in terms of other SI base units (mass, time, temperature, amount of substance, electric current, luminous intensity). It is independently defined and serves as a foundation for deriving other units (e.g., area in m², volume in m³, velocity in m/s). (d) Conversions: 5 km = 5 × 1000 m = 5000 m 5 km = 5 × 1,000,000 mm = 5,000,000 mm (or 5 × 10⁶ mm) **Q2.** (a) Classify the following motions as rectilinear, circular, or periodic: (i) A bus on a straight highway, (ii) A ceiling fan blade, (iii) A person on a swing, (iv) A satellite orbiting Earth. (b) For each, justify your classification using the defining characteristics of each motion type. (c) Can a motion be both circular and periodic? Provide a worked example. **Full Solution:** (a) Classifications: (i) A bus on a straight highway → Rectilinear motion (ii) A ceiling fan blade → Circular and periodic motion (iii) A person on a swing → Periodic motion (not circular; the path is an arc) (iv) A satellite orbiting Earth → Circular and periodic motion (b) Justifications: - Rectilinear: Motion along a single straight line with no deviation. The bus travels back and forth on a highway, always following the same linear path. - Circular: Motion along a circular arc or closed circular path. The fan blade tip traces a circle around the fan's axis; a satellite's orbit approximates a circle around Earth. - Periodic: Motion repeating at regular time intervals. A fan blade returns to the same orientation every (1/frequency) seconds; a swing reaches maximum displacement, returns to equilibrium, and repeats with a constant period. (c) Yes, a motion can be both circular and periodic. Example: A satellite in a stable orbit around Earth moves in a circle (circular motion) and completes one orbit every 90 minutes (periodic motion with period T = 90 min). Similarly, a ceiling fan blade rotates along a circle and repeats this rotation at regular intervals (e.g., 30 times per second if operating at 1800 RPM), making it both circular and periodic. **Q3.** (a) Define speed and explain the formula v = d/t. (b) A cyclist travels 15 km in the first hour, 18 km in the second hour, and 12 km in the third hour. Calculate: (i) Speed in each hour (in km/h and m/s), (ii) Average speed for the entire journey (in both units). (c) Why is average speed different from the average of individual speeds? Provide a numerical justification. **Full Solution:** (a) Speed is the distance travelled per unit time, calculated as v = d/t, where v is speed, d is distance, and t is time. Speed is a scalar quantity (magnitude only, no direction) and always positive. The formula indicates that speed increases with greater distance or shorter time intervals. (b) Speeds in each hour: Hour 1: v₁ = 15 km/h = 15 × (5/18) ≈ 4.17 m/s Hour 2: v₂ = 18 km/h = 18 × (5/18) = 5 m/s Hour 3: v₃ = 12 km/h = 12 × (5/18) ≈ 3.33 m/s Average speed for the journey: Total distance = 15 + 18 + 12 = 45 km Total time = 3 hours Average speed = 45/3 = 15 km/h = 15 × (5/18) ≈ 4.17 m/s (c) Comparison: Average of individual speeds = (15 + 18 + 12)/3 = 45/3 = 15 km/h Average speed for the journey = Total distance / Total time = 45/3 = 15 km/h In this example, they coincide because the time intervals (1 hour each) are equal. However, if time intervals differ, they diverge. For instance, if the cyclist travels 15 km in 1 hour, 18 km in 2 hours, and 12 km in 1 hour: Average of individual speeds = (15 + 9 + 12)/3 = 12 km/h [Here, speed in hour 2 is 18/2 = 9 km/h] Average speed = (15 + 18 + 12)/(1 + 2 + 1) = 45/4 = 11.25 km/h They differ because average speed is always defined as total distance ÷ total time, and unequal time intervals weight the speeds differently.

HOTS questions integrate multiple concepts, real-world scenarios, and analytical reasoning — mirroring recent CBSE board trends. **Case Study Question:** A research team is studying animal locomotion. They observe three animals: (1) a cheetah sprinting straight across an open field, (2) a falcon circling overhead, and (3) a firefly flashing on and off as it flies. **Part A:** Classify each motion type (rectilinear, circular, or periodic). **Part B:** The cheetah covers 200 metres in 8 seconds. Calculate its speed in m/s and km/h. If the cheetah maintains this speed, how far will it travel in 1 minute? **Part C:** The falcon completes one full circle (circumference 628 metres) in 20 seconds. Find its speed. How does this speed compare to the cheetah's? **Part D:** The firefly's flash has a period of 2 seconds (on for 1 second, off for 1 second). If the researcher observes for 10 seconds, how many complete flash cycles occur? Why is this motion classified as periodic despite the firefly's overall forward movement? **Solution Steps:** **Part A:** - Cheetah → Rectilinear motion (straight-line motion) - Falcon → Circular motion (circular path overhead) - Firefly → Periodic motion (flashing repeats at regular 2-second intervals); note that the firefly also exhibits rectilinear or arbitrary motion during flight, but the flashing pattern is periodic **Part B:** Cheetah's speed: v = d/t = 200/8 = 25 m/s Convert to km/h: 25 m/s × 3.6 = 90 km/h Distance in 1 minute (60 seconds): d = v × t = 25 × 60 = 1500 metres = 1.5 km **Part C:** Falcon's speed: v = d/t = 628/20 = 31.4 m/s Comparison: Falcon's speed (31.4 m/s) > Cheetah's speed (25 m/s). The falcon is 1.26 times faster. This illustrates that circulatory motion (even at constant speed) can appear faster than rectilinear motion if the trajectory covers greater distance per unit time — though here, the falcon genuinely moves faster. **Part D:** Period = 2 seconds per cycle Number of complete cycles in 10 seconds = 10/2 = 5 complete cycles Although the firefly moves forward (rectilinear or arbitrary path), its brightness-based signal repeats every 2 seconds, making it periodic in terms of light emission. This demonstrates that periodic classification refers to the repetition of a specific attribute (in this case, luminosity) over fixed time intervals, independent of spatial motion. **Key Learning:** This case study integrates motion classification, speed calculation, unit conversion, and conceptual analysis — all essential board-level competencies.

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